1. Technical Field
The present invention generally relates to cellular radio networks, and in particular relates to sharing of a radio access network having overlapping coverage with one or more unshared radio access networks.
2. Background
A known approach for reducing the investment and cost of operating a cellular radio network involves the sharing of network resources. Rather than each building a full network, two (or more) operators share a common set of base stations, radio equipment and radio frequencies. In some cases, this shared network at least partly overlays one or more other networks. For example, two operators might share a 3rd-generation (3G) network in a particular region, while each operates its own unshared GSM network in the same area. In this example, the shared network uses a different radio access technology than the overlapping networks, e.g. Wideband CDMA (W-CDMA) versus GSM, but this is not necessarily the case.
One network sharing scenario is illustrated in FIG. 1. In this scenario, operators A and B operate radio access network (RAN) 110 and RAN 120, respectively. The cell coverage of RAN 110 and RAN 120 is pictured in FIG. 1; the two networks in this example cover generally the same area. In this scenario, operators A and B agree to share a single Long-Term Evolution (LTE) network, rather than each building its own separate network. (“LTE” refers to advanced 3rd-generation network standards under development by the 3rd-Generation Partnership Project.) The shared LTE network 130 is also pictured in FIG. 1; LTE network 130 also covers generally the same area as unshared RAN 110 and unshared RAN 120. LTE-capable terminals subscribing to either operator A's service or operator B's service are permitted to access services using shared LTE network 130.
In order to provide seamless coverage for subscribers, it is important that a shared radio access network integrate smoothly with legacy, i.e. pre-existing, radio access networks. In the example illustrated in FIG. 1, operator A would like its subscribers to perform handovers between the shared LTE network 130 and its unshared GSM network 110 whenever appropriate. Likewise, operator B requires appropriate handovers between the shared LTE network 130 and operator B's unshared GSM network 120 for its subscribers. Thus, a shared radio access network should provide interoperability between the shared RAN and unshared RANs having overlapping coverage.
One approach to supporting handovers from the shared RAN 130 to multiple unshared RANs is to configure the common control channel in the shared RAN 130 with neighbor cell lists and other radio-network design parameters relating to both of the unshared networks 110 and 120 as well as the shared network 130. Thus, a mobile terminal connected to LTE network 130 would receive neighbor cell information for neighboring cells in the LTE network 130, unshared RAN 110, and unshared RAN 120. This mobile terminal will then include the neighbor cells from all three networks in its normal scanning procedures, even though some of those neighbors are in an unshared RAN that the mobile terminal is not permitted to access.
This approach is illustrated in FIG. 2. LTE base station 210 provides coverage for LTE cell 215, which has overlapping coverage with cell 220, which is part of Public Land Mobile Network (PLMN) A, and cell 225, which is part of PLMN-B. PLMN-A and PLMN-B are operated by operators A and B respectively; mobile terminals operating in the area are affiliated with one operator or the other. As with the scenario illustrated in FIG. 1, operators A and B have agreed to share the resources of the LTE network. Thus, subscribers of both operators have access to the services provided by LTE base station 210 when in the coverage area. Base station 210 and LTE cell 215 are part of a shared RAN; PLMN-A and PLMN-B are unshared RANs.
Base station 210 transmits LTE radio signal 240, which carries control channel signal 250 as well as one or more traffic channel signals. In the scenario illustrated in FIG. 2, control channel signal 250 includes neighbor cell data corresponding to LTE neighbor cells as well as to cell 220 and cell 230. In certain circumstances, a mobile terminal accessing LTE cell 215 might require handover to an underlying GSM cell—if the mobile terminal is a subscriber of operator A (or to an affiliated operator) it will be handed off to cell 220; if a subscriber of operator B it will be handed off to cell 230. Because the control channel signal 250 identifies both of these cells as neighbors, i.e. potential handoff targets, the mobile terminal will have up-to-date information for each target when handoff is needed.
However, this approach has several limitations. First, because all mobile terminals receive the same control channel information, the idle-mode behavior of mobiles belonging to operator A cannot be controlled independently of the idle-mode behavior of mobiles belonging to operator B. For example, discontinuous receive (DRX) cycles cannot be set differently for mobile terminals affiliated with the different operators.
Second, neighbor cell identification information for GSM cells belonging to network A is broadcast on the LTE common control channel, and those cells are scanned by mobile terminals belonging to network B, even though these mobile terminals will not be handed off to any of those cells. Likewise, mobile terminals belonging to network A will receive control channel information identifying neighbor cells in network B. This leads to an unnecessarily large number of neighbor cells for the terminals to track and measure. Because the quality of signal strength measurements is a function of the time devoted to the measurements, this in turn leads to unnecessarily degraded measurement information.
With this approach, other cell parameters, such as signal strength thresholds indicating when to start measurements on GSM cells, are also common for all terminals and cannot be set by operator A and B separately. Depending on the design of the underlying networks, the cell parameters may be optimal for only one of the underlying networks, or for neither. This can lead to poor radio performance, and in some cases can lead to an increase in dropped calls.
In addition, the two operators may desire different criteria for when a mobile terminal should leave the LTE network in favor of the respective GSM network, depending on, for example, pricing terms for using the shared network, coverage and performance of the legacy networks, available services, and so on. Likewise, at any given time the two underlying networks may be experiencing dramatically different loading conditions, so that the operators have different objectives with respect to balancing loads between the LTE network and the underlying GSM networks. This fine tuning of network performance is not possible with a common set of radio-network parameters in the common control channel.
Finally, control channel information in the scenario of FIG. 2 also dictates identical behavior for all receiving mobile terminals with respect to the LTE network. This creates challenges for a scenario where only part of the LTE network is shared. One such scenario arises when only macro cells of the LTE network are intended to be shared, while only mobile terminals associated with one operator are allowed access to micro cells. In this scenario, the micro cells make up an unshared RAN, even though the micro cells are part of the same physical network as the macro cells. However, because the common control channel information is shared by all mobile terminals, regulating access to those micro cells is difficult.
Another approach to sharing physical network resources that addresses some of the above problems is to configure two independent cells in the same radio base station. This approach is pictured in FIG. 3. LTE base station 310 transmits a signal comprising parts 320 and 325, each covering 10 MHz of the total 20 MHz available. Part 320 comprises a first control channel signal 330 and corresponding traffic channel signals for users associated with operator A, while part 325 comprises a second control channel signal 335 and corresponding traffic channel signals for users associated with operator B. Thus, a single base station 310 simultaneously serves two independent LTE cells 340 and 345, which overlay GSM cells 350 and 355.
Because this approach results in two distinct control channels signals 320 and 325, operation of mobile terminals associated with operators A and B may be controlled independently. While this approach addresses the issues of idle-mode behavior and cell selection, the result is that there is incomplete sharing of the network resources, since operators A and B can each use only half of the available LTE spectrum. This leads to non-optimal peak rate performance.